In order to secure power per bit, which has been increasing as the data transmission speed becomes higher, it is useful to reduce the loss in a feed system (see Non-Patent Literature 1, for example). FIG. 20 illustrates a configuration in which a transmitting and receiving front end circuit and an antenna are integrally implemented, which is one of the means for implementing the reduction of loss in a feed system.
In FIG. 20, a diplexer 103, which separates transmitting and receiving bands, is provided immediately below an antenna 101, and bandpass filters 105 and 107 for removing unnecessary frequency band signals are provided in a stage subsequent to the diplexer 103. A low noise amplifier (LNA) 109, which is arranged in a stage subsequent to the bandpass filter 105, and a power amplifier (PA) 111, which is arranged in a stage subsequent to the bandpass filter 107, are provided in order to increase the level of signals in the receiving band and the transmitting band, respectively.
According to the above-described configuration, the reduction of a noise figure (NF) can be implemented for the receiving band, and the reduction of required radiation power can be implemented for the transmitting band. However, if the above-described configuration is applied to a frequency division duplex (FDD) system, a required value becomes high for the bandstop amount for signals in unnecessary frequency bands in order to separate signals in the transmitting band, and accordingly, a problem may arise such that it becomes necessary to respond to the high required value by using a multi-stage large diplexers and bandpass filters.
Under these circumstances, in order to implement a small-sized filter, the transmitting-receiving-separated antennas according to Patent Literatures 1 to 3 have been proposed, which additionally include function of a diplexer. The transmitting-receiving-separated antennas implement reduction of the number of stages of filters arranged at stages subsequent to the antenna by suppressing cross coupling between an antenna for the transmitting band and an antenna for the receiving band (this phenomenon will be hereafter referred to as “coupling between transmission and reception”). The antenna according to Patent Literature 1 reduces coupling between transmission and reception in a configuration in which polarized waves in the transmitting and those in the receiving band are crossed one another, and coupling between transmission and reception as low as about −35 dB can be achieved thereby. The antenna according to Patent Literature 2 reduces coupling between transmission and reception in the transmitting band, even among the same polarized waves, to a level as low as about −30 dB by providing a parasitic element intended as a bandstop. The antenna according to Patent Literature 3 reduces coupling between transmission and reception to a level as low as about −50 dB by using circular polarization antennas of the same nutation direction as antennas for the transmitting band and the receiving band and by allowing one element structure to rotate.
The above-described antennas according to Patent Literatures 1 and 2 have a basic structure as a transmitting-receiving-separated patch antenna which uses a microstrip line. Now, coupling between transmission and reception occurring in the case in which polarized waves are shared in a transmitting-receiving-separated patch antenna described above will be discussed. FIG. 21 is a perspective view which illustrates a transmitting-receiving-separated dual-polarization patch antenna which uses a microstrip line as a feed line, and FIG. 22 is an exploded view of the patch antenna. The transmitting-receiving-separated dual-polarization patch antenna includes a patch antenna 201-T for a transmitting band and a patch antenna 201-R for a receiving band. The patch antenna 201-T for transmitting band is constituted by a patch 203, a cross-shaped feed slot 205, which is located below the patch 203, and feed lines 207a and 207b, which are located below the feed slot 205. The patch antenna 201-R has a similar configuration. The feed slot 205 of the patch antenna 201-T, 201-R is formed on a common ground conductor plate 209. In addition, a dielectric substrate 211 is arranged between the ground conductor plate 209 and the feed line 207a, 207b. Note that the transmitting-receiving-separated dual-polarization patch antenna does not include any configuration for implementing the bandstop, which is included in the antenna according to Patent Literature 2.
In the transmitting-receiving-separated dual-polarization patch antenna, power is fed to the patch 203 of the patch antenna 201-T for the transmitting band so that polarized waves are shared by electromagnetic coupling with the feed line 207a, 207b via the corresponding feed slot 205. The patch 203 of the patch antenna 201-R for the receiving band has a similar configuration. FIG. 23 illustrates a characteristic of coupling between transmission and reception with respect to the transmitting-receiving-separated dual-polarization patch antenna. In this drawing, fR denotes a center frequency of the receiving band, fT denotes a center frequency of the transmitting band, and f0 denotes a center frequency between a lower limit frequency of the receiving band and an upper limit frequency of the transmitting band, respectively, and fR=0.953 f0, fT=1.047 f0.
As is clear from the coupling characteristic, according to the transmitting-receiving-separated dual-polarization patch antenna, coupling between transmission and reception among cross polarized waves (i.e., among vertically polarized waves for the receiving band and horizontally polarized waves for the transmitting band, and among horizontally polarized waves for the receiving band and vertically polarized waves for the transmitting band) can be reduced to −40 dB or lower, however, with respect to the coupling between transmission and reception among the same polarized waves (i.e., among vertically polarized waves for the receiving band and vertically polarized waves for the transmitting band, and among horizontally polarized waves for the receiving band and horizontally polarized waves for the transmitting band), the worst case value becomes −20 dB or higher. As described above, in the transmitting-receiving-separated dual-polarization patch antenna described above, coupling among the same polarized waves becomes intense.